Spin-related Cu-Co pair to increase electrochemical ammonia generation on high-entropy oxides

The electrochemical conversion of nitrate to ammonia is a way to eliminate nitrate pollutant in water. Cu-Co synergistic effect was found to produce excellent performance in ammonia generation. However, few studies have focused on this effect in high-entropy oxides. Here, we report the spin-related Cu-Co synergistic effect on electrochemical nitrate-to-ammonia conversion using high-entropy oxide Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O. In contrast, the Li-incorporated MgCoNiCuZnO exhibits inferior performance. By correlating the electronic structure, we found that the Co spin states are crucial for the Cu-Co synergistic effect for ammonia generation. The Cu-Co pair with a high spin Co in Mg0.2Co0.2Ni0.2Cu0.2Zn0.2O can facilitate ammonia generation, while a low spin Co in Li-incorporated MgCoNiCuZnO decreases the Cu-Co synergistic effect on ammonia generation. These findings offer important insights in employing the synergistic effect and spin states inside for selective catalysis. It also indicates the generality of the magnetic effect in ammonia synthesis between electrocatalysis and thermal catalysis.

Supplementary Fig. 14 The M-H curves of RS-0, RS-20, and Li-RS-16 at room temperature.a Magnetic moment normalized by mass.b Magnetic moment normalized by mole number.
We conducted the VSM test at room temperature using Lake Shore 7400 VSM.Supplementary Fig. 14 shows the M-H curves of RS-0, RS-20 and Li-RS-16, where M is the magnetization, H is the magnetic field strength.The magnetic susceptibility χ can be obtained based on the equation M = χH.
The linear M-H relationship and the positive χ values of RS-0, RS-20, Li-RS-16 indicate that these three samples are paramagnetic at room temperature.From the relationship χ = Ng 2 J(J+1)μB 2 / (3kBT), where N, g, J, μB, kB and T are the Avogadro number, g-factor, angular momentum quantum number, Bohr magneton, Boltzmann constant and temperature, respectively.A larger χ value means a larger J, indicating a higher spin state.The M-H curves shows that RS-0 and RS-20 have a very close magnetic susceptibility.Li-RS-16 has a smaller magnetic moment than RS-0 and RS-20.The total mole fractions of Co, Ni and Cu in RS-0, RS-20 and Li-RS-16 are 0.50, 0.60 and 0.48, respectively.If the magnetic moment contribution is considered to merely from the Co, Ni and Cu, the magnetic moments of RS-0 and RS-20 are still higher than that of Li-RS-16.However, as the electronic states Apart from VSM, we also used soft X-ray to characterize the Co, Ni and Cu L-edges and O K-edge of RS-0, RS-20, and Li-RS-16 (Supplementary Fig. 15).Soft X-ray absorption experiment was performed at the SUV beamline of Singapore Synchrotron Light Source (SSLS).The L2,3-edges of Co, Ni, and Cu represent the transition from 2p 6 3d n to 2p 5 3d n+1 , where n is the electron number in the d orbitals.Take Co as an example, lower energy L3 peak stands for the transition 2p3/2 → 3d and higher energy L2 peak stands for the transition 2p1/2 → 3d 1 .The Co L3 peak of RS-20 and RS-20 splits into two peaks at 780.8 eV and 782.0 eV with similar weight.The Co L3 peak of Li-RS-16 and RS-20 splits into a weak peak at 780.8 eV and a strong peak 782.5 eV.Although the interpretation of Co Ledge requires the consideration of multiplet splitting, hybridization, and crystal field effects, here we simplify this behavior as energy splitting between the t2g states and the eg states 2 .In the Oh symmetry, Co 2+ has the low spin state t2g 6 eg 1 and high spin state t2g 5 eg 2 , and Co 3+ has the low spin state t2g 6 eg 0 , intermediate spin state t2g 5 eg 1 , and high spin state t2g 4 eg 2 .Therefore, the peak intensity theoretically corresponds to the hole number of t2g (at low energy) and eg (at high energy).The Co L3-edges of RS-0 and RS-20 have no obvious change, indicating no electron occupation change.When comparing Li-RS-16 and RS-20, the peak corresponding to eg orbitals shifts to the high energy direction relative to RS-20.This indicates that the Co valence state increases, in agreement with our XANES results and the previous investigation 3 .In Li-RS-16, the peak corresponding to eg is much stronger than the peak corresponding to t2g, indicating that there are less holes in t2g orbitals and more holes in eg orbitals.For Co 2+ or Co 3+ , it suggests there are less unpaired electrons and exhibits lower spin.Besides, there is a larger peak splitting energy between t2g and eg orbitals in Li-RS-16 than in RS-0 and RS-20, which is consistent with EXAFS results that Co-O distance is smaller in Li-RS-16 than in RS-0 and RS-20.A smaller Co-O causes a strong crystal field that induces a larger t2g and eg splitting.
For Ni and Cu cations, we did not observe the obvious difference among RS-0, RS-20 and Li-RS-16.Since the O K-edge represents the electronic transitions from the O 1s core level to the unoccupied transition metal 3d levels with the O 2p components, due to the coexistence of multiple cations, we cannot use it to analyse the d-orbital occupation of cations.A remarkable difference is that the signal reflecting the hole in oxygen (at ~531.5 eV) becomes stronger from RS-0, RS-20 to Li-RS-16, which is consistent with our EELS results.Another difference is the disappearance of the peak at 533.2 eV representing the metal-oxygen covalency 4 , which could be due to the highly ionic Li-O bond.

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Ni and Cu are not observed remarkably different, the magnetic moment decreases could come from the decrease in Co spin state.Supplementary Fig. 15 XAS of RS-0, RS-20, and Li-RS-16.a Co L-edge.b Ni L-edge.c Cu Ledge.d O K-edge.